Evaluation of a long‐acting injectable formulation of omeprazole in healthy dogs

Abstract Background To evaluate the efficacy of a single intramuscular adminsitration of long‐acting omeprazole (LA‐OMEP) in increasing gastric pH in dogs. Hypothesis We hypothesized that LA‐OMEP would meet in healthy dogs the clinical goals defined for human patients for treatment of gastroduodenal ulceration. Animals Nine healthy research dogs. Methods Prospective experimental study. Dogs were given a 4 mg/kg intramuscular injection of LA‐OMEP. Intragastric pH was continuously recorded on treatment days 0 to 7. Daily mean pH and mean percentage time (MPT) intragastric pH was ≥3 or ≥4 were determined. Results The mean onset of action for the LA‐OMEP was 98.11 min (SD 46.39). The mean number of days the dogs' pH met established goals for MPT pH ≥3 was 5.5 days (range, 3‐7) and 5.25 days for MPT pH ≥4 (range, 3‐7). Long‐acting omeprazole met the human clinical goals pH ≥3 for 72 hours in 8/8 of the dogs and MPT pH ≥4 for 96 hours in 7/8 of dogs. Conclusions and Clinical Importance The LA‐OMEP formulation produced gastric acid suppression in healthy dogs for an average of 5 days and up to 7 days, after a single intramuscular injection. No major adverse effects were observed.


| INTRODUCTION
Gastrointestinal (GI) bleeding is common in critically ill dogs and is a cause of increased morbidity and case fatality. 1,2 Clinical signs of GI bleeding include anorexia, lethargy, vomiting, abdominal pain, hematemesis, melena, and hematochezia. 3,4 Central to the treatment of upper GI bleeding is inhibition of gastric acid suppression through the administration of gastric acid-suppressant treatment. 1,4,5 In human patients, the use of proton pump inhibitors (PPIs) reduces both bleeding risk and the need for endoscopic intervention in bleeding patients. 4,6 Suppressing gastric acid production and maintaining intragastric pH ≥3 and ≥4 for 75% and 67% of the day promotes the healing of duodenal ulcers and gastroesophageal reflux disease in human patients, respectively. 7,8 In dogs, PO and IV administation of PPIs are superior to standard doses of histamine-2 receptor antagonists in increasing intragastric Abbreviations: GI, gastrointestinal; LA-OMEP, long-acting injectable omeprazole; MPT, mean percentage time; PPI, proton pump inhibitor; RAH, rebound acid hypersecretion. pH for treating gastric/duodenal ulceration as well as moderate to severe esophagitis. 9,10 However, although there are no established clinical goals for acid suppression in dogs, IV administration of pantoprazole to dogs for treatment of duodenal ulceration does not consistently meet the aforementioned clinical goals established for humans. 10,11 Recently, long-acting injectable omeprazole (LA-OMEP) was evaluated in horses (100 mg/mL, Luoda Pharma, Caringbah, NSW, Australia). 12,13 This novel omeprazole product, suspended in a vehicle selected for tissue tolerance, provided acid suppression in horses after a single injection for up to 7 days. The single injection met the clinical goals for acid suppression defined in people in 57% (4/7) of horses for all 7 days and 100% (7/7) of horses for 4 days. 12 A potential advantage of this formulation in dogs would be sustained acid suppression after a single injection and increased treatment compliance for dogs with severe upper GI bleeding as missing even 1 dose of an oral PPI could have deleterious effects on maintaining desired acid suppression. 14 The LA-OMEP formulation could also potentially reduce the duration of hospitalization in affected dogs.
The study objective was to evaluate the efficacy of LA-OMEP in increasing intragastric pH in dogs after a single intramuscular injection. We hypothesized that LA-OMEP would meet the clinical goals defined for human patients in our cohort of healthy dogs.

| Study animals
The study protocol was approved by the Institutional Animal Care and Use Committee at the University of Tennessee. Nine adult healthy purpose-bred Beagle dogs from a research colony at the University of Tennessee were enrolled in the study (4 spayed females, 5 castrated males), all approximately 5 years of age and weighing 9 to 14.1 kg (median 12.3 kg).
The number of dogs was selected based on a sample size calculation using the study that evaluated LA-OMEP in horses. 12 In order to detect a 20% change in mean percentage time (MPT) pH is ≥4, using a conservative SD of 16.07 and assuming a moderate correlation of 0.6 and an alpha of 0.05, 7 dogs were needed to have an 80% power of finding significant differences over time, if they existed. Two additional dogs were enrolled to account for potential study dropout.
All dogs were deemed healthy based on a physical exam performed at the beginning of the study and recent diagnostic tests (CBC, serum biochemistry, urinalysis, and a fecal examination). The dogs were maintained in a closed colony and received monthly preventative care, including an anthelmintic. In order to detect any adverse effects from LA-OMEP, the dogs were monitored for inappetence (characterized by consuming <50% of their meals on more than 3 consecutive occasions), weight loss >10% of their body mass, >3 episodes of vomiting in a 24-hour period or diarrhea characterized by a Purina fecal score >/=5 for more than a 48-hour period. The dogs were fed their normal commercial dry food diet, Purina One Smart Blend Lamb and Rice Formula (Nestlé Purina PetCare Company, St. Louis, Missouri) once daily in the morning and water was given ad libitum throughout the study. The injection site was also monitored 3 times a day for adverse effects and was graded using a modification of a previously published grading system evaluating for pain, tenderness, swelling, necrosis, and ulceration. 15

| Intragastric pH capsule placement
On the morning of day 0, the morning meal was withheld and the dogs were sedated using butorphanol (0.2 mg/kg IV; Torbugesic 10 mg/mL injection; Fort Dodge Animal Health, FortDodge, Iowa) and dexmedetomidine (5-10 μg/kg IV; Dexdomitor 0.5 mg/mL injection; Orion Pharma, Espoo, Finland). The Bravo pH capsules (Medtronic, Minneapolis, Minnesota) were placed utilizing digital radiology for assistance. Intragastric pH capsules were adhered to the dogs' gastric mucosa as described in other studies. 11 After the procedure, the sedation was reversed with an equal volume of atipamezole (0.05-0.1 mg/ kg IM; Antisedan 5 mg/mL injection; Orion Pharma).

| Study design
A prospective, experimental nonrandomized study was designed.
After the pH capsule was placed on the morning of day 0, baseline intragastric pH data were collected the rest of that day. On day 1, all dogs received 4 mg/kg of the 100 mg/mL LA-OMEP formulation

| Intragastric pH monitoring
Intragastric pH was recorded continuously for at least 8 days after capsule placement starting on day 0 and continuing through day 7 or until the capsule detached, if detachment occurred before day 7. The Bravo pH capsule naturally detaches from the gastric mucosa within 2 to 4 days after placement. A new capsule was placed in each dog on day 3, and on any day capsule detachment occurred before the end of day 7. Intragastric pH was continued to be monitored if the pH capsule remained attached to the gastric mucosa after day 7. Telemetric data from the capsules were transferred to a corresponding recorder that was placed within 3 feet or 1 m of the dog and remained with the dog throughout the observation period. pH data were uploaded to a commercial computer software system (Reflux Software v6.1, Medtronic) every 24 hours. The receiver was then reset after the data were uploaded, and the same receiver was used to capture the next 24 hours of data. A right lateral radiograph was taken to confirm the pH capsule was still within the stomach if early gastric detachment and passage was suspected based on a rapid and sustained increase in the intragastric pH >4. The onset of action of LA-OMEP in each dog was determined by the time after which the intragastric pH was persistently >4 for at least 60 minutes.
One dog had only 1 capsule placed and was terminated from the study due to a seizure (see adverse effects). Two dogs required 3 capsules for the study duration and 6 dogs required 4 capsules for the study duration. When capsule displacement occurred, the data available until the time of detachment were retained for analysis. Most capsules were placed on days 0, 3, and 7. All capsules were replaced between 44 minutes to 8 hours and 33 minutes of pH capsule detachment from the gastric mucosa.

| Intragastric pH recording
The mean intragastric pH and the MPT intragastric pH were ≥3 and ≥4 are graphically depicted in Figures 1 to 3, respectively.
Intragastric pH on day 0 was significantly lower than all days including day 7 (P < .001, for each). There was no difference observed between days 1 and 5. Intragastric pH on day 6 was significantly lower than days 1 to 4 (P ≤ .03, for each) but did not differ from day 5. Intragastric pH on day 7 was significantly lower than days 1 to 5 (P ≤ .02, for each) but did not differ from day 6.
When evaluating MPT pH ≥3, an overall mean difference was observed between days (P < .001). Intragastric pH on day 0 was significantly lower than all other days, including day 7 (P < .001, for each). Intragastric pH on day 7 was significantly lower than days 1 to 4 (P ≤ .002, for each) but did not differ from days 5 or 6. No difference was observed between days 1 to 6 or days 5 to 7.
For MPT pH ≥4, an overall mean difference was observed between days (P < .001). Intragastric pH on day 0 was significantly lower than all other days, including day 7 (P ≤ .001, for each).
Intragastric pH on day 6 was significantly lower than days 1 to 3 (P ≤ .04, for each) but did not differ from days 4 to 5. Intragastric pH on day 7 was significantly lower than days 1 to 5 (P ≤ .05, for each) but did not differ from day 6.
The mean number of days the dogs' pH met established goals for MPT pH ≥3 was 5.5 days and 5.25 days for MPT pH ≥4. Four dogs had pH recordings captured after day 7. These data were not included in the statistical review due to the small amount of data collected.

| ADVERSE EFFECTS
Eight dogs completed the study. One female dog was removed from the study due to a single seizure. That dog had a grand mal seizure on day 1, about an hour after the LA-OMEP was administered. A physical and neurologic examination (performed by the neurology service) did not detect abnormalities. Serum electrolyte and blood glucose concentration were all within the normal reference range. The dog was monitored overnight and did not have any additional seizures. The pH data from that dog were captured through day 3 until the pH capsule detached from the gastric mucosa. The dog was excluded from completing the study beyond this point.
There were 3 episodes of vomiting by 3 dogs. These vomiting episodes happened on days 1, 6, and 7. Two of the vomiting episodes happened on days 6 and 7, after the dogs were recovering from the pH capsule placement. There were 4 episodes of hyporexia where <50% of the food was consumed by 3 dogs. All the episodes of hyporexia were associated with sedation on pH capsule placement days. The median fecal score was 2.5 (1-4.6). There were 7 episodes, from 6 dogs, where the fecal score was >/=5. All episodes where the fecal score was >/=5 were associated with sedation on pH capsule placement days.
There were no injection site adverse effects noted in any dog on any day. No dog lost >10% of their baseline bodyweight during the study. Results from a complete blood count and serum biochemistry panel obtained 2 weeks after the study did not reveal any abnormalities.  16 The PPIs and other gastroprotectants are often used inappropriately and overprescribed by general practitioners and veterinary specialists. [17][18][19] The authors advise judicious use of this medication for dogs where severe GI bleeding, treatment compliance, or both is a concern.

| DISCUSSION
There was individual variability in response to LA-OMEP demon- F I G U R E 3 The mean percentage time (MPT) intragastric pH ≥4 on treatment days 1 to 7 for all dogs receiving a single dose of 4 mg/ kg long-acting omeprazole intramuscularly. The shaded dot, horizontal and vertical lines represent the individual dog mean, group mean and standard deviations, respectively. Significant increases in MPT intragastric pH ≥4 were noted between days 0 and all other days (P ≤ 0.001 for each) variation seen in the present study is still undetermined but was also observed in a study evaluating LA-OMEP in horses. 12  Thus, LA-OMEP confers the additional advantage of a rapid onset of action in critically ill dogs with profound GI bleeding.
The authors decided to use the epaxial muscle bed as the injection site due to ease of observing injection site reactions with minimal disruption of the dogs. However, 1 study demonstrated that the fastest absorption of dexmedetomidine and hydromorphone was achieved after injection into the semimembranosus muscle. 19 It is difficult to say if absorption pharmacokinetics will be similar for LA-OMEP, however an average onset of action of 98.11 minutes is still timely for most critically ill dogs with GI bleeding or moderate to severe esophagitis.
The LA-OMEP dose used in the present study, 4 mg/kg IM as an extrapolation from the equine studies, appears to be an effective and safe dose in dogs. Additional studies evaluating higher or lower doses, in terms of duration of action and efficacy, might be considered in the future.
Rebound acid hypersecretion (RAH), a phenomenon in which hypergastrinemia caused by prolonged drug-induced inhibition of gastric acid secretion results in acid hypersecretion after discontinuation of acid suppressant treatment, occurs in human patients with a history of extended use of acid suppressants and in cats after 60 days of omeprazole treatment. [21][22][23] It is estimated that long-term (usually 3 months or longer, although it could happen after 28 days of treatment) PPI treatment causes moderate hypergastrinemia and rebound hypersecretion in about 30% to 40% of human patients when PPIs are abruptly discontinued. 22,24,25 This leads to symptoms of gastroesophageal reflux (heartburn, regurgitation, and burning sensation in the esophagus) in humans. Recommendations are made for humans to wean off PPIs in patients who have received them for an extended time or switch them to a less effective acid blocker (eg, histamine-2 receptor antagonists). 22 The incidence of RAH is unknown in dogs. In a study of dogs receiving 2 weeks of famotidine treatment, serum gastrin levels were increased after 3 days of famotidine administration but decreased in most of the dogs by day 12 of treatment. 26 Similar results were found in another study evaluating gastrin levels in dogs treated with famotidine for 14 days. 27 Although LA-OMEP produced potent intragastric acid suppression in the dogs in the present study, on average, its effect decreased slowly over time. On average, the mean intragastric pH and MPT pH was still higher than baseline on day 7. Future studies are needed to evaluate how quickly the intragastric pH returns to baseline in dogs. However, RAH should be considered in dogs treated with LA-OMEP with 1 or more doses, although the benefit with repeated dosing is currently undetermined.
The software tracing utilized in this study makes it easy to determine if the pH capsule is still in the stomach. Despite excellent intragastric acid suppression noted in the present study, when the capsule is located in the stomach, multiple acid spikes can be observed during the day with the intragastric pH approaching <4 for periods of time.
When the capsule moves into the small intestine, the pH tracing stays persistently higher than pH 4 without acid spikes. This observation prompts an abdominal radiograph to investigate for capsule migration.
LA-OMEP-related adverse effects were minimal in the present study. All episodes of vomiting, hyporexia, and diarrhea were associated with the days the dogs were sedated for capsule placement. All adverse effects were resolved within 24 hours of sedation for all dogs.
One dog had a seizure shortly after the LA-OMEP injection. Diagnostic testing and a neurologic exam suggested idiopathic epilepsy, although an MRI or a CSF tap were not performed. Further investigation revealed a history of seizures in the lineage of that dog. Although the dog did not complete the study, no additional seizures occurred during the duration of the study. The dog was reported to have a single seizure about 3 months after the study was completed, which is more consistent with idiopathic epilepsy.
This study had an unusually high frequency of capsule failure during placement. The manufacturers had a recall of capsules because of similar issues and also sent out an urgent "Field Safety Alert" after the present study was completed, notifying users of similar feedback of a high capsule failure rate. Although the capsules used in the present study were not part of the recalled lot, it is likely that a defect in capsule manufacture led to the high failure rate. Although the Bravo pH technology allows data recording for 72 to 96 hours, the authors decided to download the data every 24 hours, in order to assess for early capsule migration to facilitate earlier replacement.
Limitations of the current study include the absence of pharmacokinetic and limited pharmacodynamic information, the limited number of dogs with pH information after day 7 and the exclusion of dogs with clinical GI bleeding or suspicion for GI disease. Additionally, a small amount of data was lost when early capsule migration occurred prior to capsule replacement. The technology records approximately 14 000 pH measurements per 24 hours and, on average, 274 measurements were lost per dog every 24 hours, with most pH being relatively stable across the data. It is unlikely that these lost data led to erroneous conclusions.